Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,184.6	$ 2,094.2
Reserves for Future Policy Contract Benefits		45,601.6	44,245.9
Unearned Premiums		373.1	363.7
Policy and Contract Benefits		1,605.2	1,507.9
Premium Income		8,597.1	8,357.7	$ 8,082.4
Net Investment Income	[1]	2,451.7	2,459.0	2,481.2
Benefits and Change in Reserves for Future Benefits	[2]	7,055.7	6,941.8	6,782.8
Amortization of Deferred Acquisition Costs		527.1	493.0	482.3
All Other Expenses	[3]	2,300.0	2,264.0	2,227.9
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,205.4	1,176.5
Reserves for Future Policy Contract Benefits		11,382.1	11,312.6
Unearned Premiums		49.0	46.9
Policy and Contract Benefits		1,059.8	977.2
Premium Income		5,443.5	5,240.9	4,960.0
Net Investment Income	[1]	811.2	828.7	865.3
Benefits and Change in Reserves for Future Benefits	[2]	3,693.4	3,624.3	3,476.7
Amortization of Deferred Acquisition Costs		293.6	275.2	272.3
All Other Expenses	[3]	1,398.0	1,369.2	1,345.5
Premiums Written	[4]	3,701.3	3,564.4	3,353.8
Unum UK
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		21.3	21.4
Reserves for Future Policy Contract Benefits		2,361.6	2,127.9
Unearned Premiums		125.7	110.3
Policy and Contract Benefits		108.5	102.4
Premium Income		513.0	529.3	576.2
Net Investment Income	[1]	120.2	118.1	124.9
Benefits and Change in Reserves for Future Benefits	[2]	381.9	367.4	394.8
Amortization of Deferred Acquisition Costs		9.1	9.7	11.3
All Other Expenses	[3]	131.2	141.9	154.4
Premiums Written	[4]	404.2	435.4	453.0
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		957.9	896.3
Reserves for Future Policy Contract Benefits		2,224.2	2,103.5
Unearned Premiums		34.7	33.5
Policy and Contract Benefits		212.1	195.8
Premium Income		1,511.4	1,417.1	1,338.6
Net Investment Income	[1]	144.9	141.5	145.4
Benefits and Change in Reserves for Future Benefits	[2]	788.6	726.4	683.0
Amortization of Deferred Acquisition Costs		224.4	208.1	198.7
All Other Expenses	[3]	331.8	311.1	293.3
Premiums Written	[4]	1,210.6	1,145.0	1,088.3
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		29,633.7	28,701.9
Unearned Premiums		163.7	173.0
Policy and Contract Benefits		224.8	232.5
Premium Income		1,129.2	1,170.4	1,207.6
Net Investment Income	[1]	1,354.0	1,352.2	1,320.0
Benefits and Change in Reserves for Future Benefits	[2]	2,191.8	2,223.7	2,228.3
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	247.3	255.4	269.5
Premiums Written	[4]	1,109.8	1,161.8	1,198.5
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income	[1]	21.4	18.5	25.6
Benefits and Change in Reserves for Future Benefits	[2]	0.0	0.0	0.0
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	191.7	186.4	165.2
Premiums Written	[4]	$ 0.0	$ 0.0	$ 0.0

[1] Net investment income is allocated based upon segmentation. Each segment has its own specifically identified assets and receives the investment income generated by those assets.

[2] Included in 2017 are unclaimed death benefits reserve increases of $26.6 million in Unum US and $12.4 million in Colonial Life.

[3] Includes commissions; interest and debt expense; deferral of acquisition costs; compensation expense; and other expenses. Where not directly attributable to a segment, expenses are generally allocated based on activity levels, time information, and usage statistics. Included in 2017 is a loss contingency accrual for a guaranty fund assessment of $20.6 million in Corporate.

[4] Excludes life insurance.